[FIX] t-out: does not crash when outputting null

Before this commit, doing `t-out="null"` would crash, since the
safeOutput function would assume that it is a block, thanks to the fact
that `typeof null === 'object'`.

We handle here the null value just as if it was undefined: it outputs an
empty string.

src/runtime/template_helpers.ts

@@ -134,7 +134,7 @@ class LazyValue {
  * Safely outputs `value` as a block depending on the nature of `value`
  */
 export function safeOutput(value: any, defaultValue?: any): ReturnType<typeof toggler> {
-  if (value === undefined) {
+  if (value === undefined || value === null) {
     return defaultValue ? toggler("default", defaultValue) : toggler("undefined", text(""));
   }
   let safeKey;